The present invention relates to disc brakes for vehicles, and in particular to a brake application arrangement for generating braking forces in disc brakes such as air-operated disc brakes utilized on commercial vehicles.
Pneumatically-operated disc brakes have been undergoing development and deployment on commercial vehicles since at least the 1970's, and are beginning to replace drum-style brakes due to advantages in areas such as cooling, fade resistance and serviceability. German Patent Publication No. DE 40 32 886 A1, and in particular FIG. 1 of this document, discloses an example of such an air disc brake. In this design, a pneumatic diaphragm chamber (pneumatic actuator) is attached to a rear face of the disc brake caliper housing, and applies a brake actuation force through a linear actuator rod to a brake actuator lever within the caliper. The brake's actuator lever in turn transfers and multiplies the force applied by the actuator rod to one or more spindles, thereby advancing brake pads against a brake disc or rotor. The terms “brake disc,” “rotor” and “brake rotor” are used interchangeably herein.
As shown in FIG. 1 of DE 40 32 886 A1, the actuator is located inboard of the brake caliper and the brake application mechanism within the caliper must be carefully arranged to have as short a height in the radial direction as possible. This is in large part because the amount of clearance between the brake caliper and the wheel rim is very tight due to commercial vehicle wheel rims in the having been sized in the past to only provide adequate clearance for the drum-type brakes historically employed on such vehicles, and because the maximum use of the available space within the wheel rim envelope permits greater brake performance to be obtained using larger brake discs.
A widely-used brake application mechanism a rotary lever arrangement such as shown in prior art FIG. 1, in which the brake actuator's operating rod 10 engages a cup at the upper end of a rotary lever 9, such that when the operating rod 10 is extended toward the rotary lever 9 (i.e., in the brake application direction) the rotary lever 9 rotates about an eccentric bearing arrangement 8 to cause brake application spindles or pistons 14 to advance a brake pad 19 toward the brake disc 1 to apply the brake. This common brake application mechanism is used in part because it can transmit the brake actuator's brake application force to the brake pad using a mechanism which is low enough in height to fit within the wheel rim envelope, while also multiplying the brake application force on the order of 15-20 times, thereby minimizing the needed brake actuator and related component sized and the necessary pneumatic pressure which must be applied to the actuator while still generating sufficiently high brake pad application forces to effectively slow the vehicle.
While the conventional rotary lever brake application mechanism has proven highly effective, their design and operation is relatively complicated (for example, high machined parts count, need to include wear adjusting devices to compensate for pad wear, etc.), and the high leverage ratios and relatively space-constrained caliper volumes result in very high component stress loadings in several locations which may adversely affect brake component design life, wear and long-term reliability.
The present invention is directed to an innovative brake application mechanism which addresses these and other issues in the prior art, while providing a robust and mechanically less complicated arrangement. In an embodiment of the present invention, the rotary lever is not supported on an eccentric bearing arrangement, but is connected to a cylindrical pinion having an axis of rotation perpendicular to the axis of rotation of the brake disc. When the rotary lever is pressed toward the brake disc by the brake actuator, the pinion rotates about its cylindrical axis. The pinion is provided with gear teeth at the opposing ends of the pinion cylinder which engage corresponding gear teeth of opposing brake pad application racks. When the rotary lever is pressed forward by the brake actuator's operating rod, the brake application mechanism's rack-and-pinion mechanism advances the opposing racks' brake pad contact faces toward the brake disc to press the inboard brake pad against the brake disc. The brake pad application racks may be generally circular in shape, and pivot about fixed rotation axes which are parallel to the brake disc and perpendicular to the axis of rotation of the pinion. The gear teeth of the racks need only extend around the portion of a circle corresponding to the range of rack rotation that would be generated by the full range of rotation of the pinion. The gear teeth may also be beveled so that the axes are aligned to prevent gear binding.
The present invention may also include a mechanism which, simultaneous with the advancing of the inboard brake pad toward the brake disc, draws the portion of the brake caliper holding the outboard brake pad toward the brake disc in order to equally and precisely displace both brake pads toward and away from the brake disc. Such a mechanism may include the outer portion of the brake caliper (aka, the outboard caliper arm) being a floating arm, i.e., a component which is slidably displaceable relative to the inboard portion of the brake caliper, and having a guide pin which interacts with at least one of the brake application mechanism's racks. Preferably the outboard portion of the caliper is provided with two guide pins having gear teeth along their lengths which engage corresponding gear teeth on the inboard racks. As the rotary lever is advanced and the racks advance the inboard brake pad toward the brake disc, the guide pins are simultaneously drawn by the racks to pull the outboard portion of the caliper and its outboard brake pad toward the opposite side of the brake disc an equal amount. The brake caliper is preferably installed in a matter which centers the brake disc between the inboard and outboard brake pads to minimize the potential for uneven application of brake pad pressure to the sides of the brake disc and consequent uneven brake pad wear, but may also be configured to accommodate a floating brake disc, i.e., a brake disc which is slidable along its axis of rotation such that the brake disc self-centers between the inboard and outboard brake pads to achieve the same objectives.
The brake application mechanism of the present invention assists in providing even brake pad wear, lowers brake caliper weight and greatly simplifies brake manufacture, assembly and maintenance in a very robust design. One of the advantages of this rack-and-pinion design is that there may be an opportunity to eliminate the prior art's costly and complicated precision wear adjustment mechanisms from the brake caliper (especially for hydraulic actuation). In addition to the cost savings, elimination of such adjusting mechanisms would also eliminate a source of undesired brake mis-adjustment caused by vibration-induced micro-motion of the adjusters.
The present invention also permits the use of a stationary caliper chamber and fixed-location rack and pinion components, thereby eliminating complex multi-component brake application mechanism assemblies which must be assembled outside of the caliper, arranged to remain assembled while being installed in the caliper, and still capable of having the entire assembly move toward/away from the brake disc during brake operation. The mechanism also may be installed as a one piece cartridge. Intent is to no longer need the sliding caliper. The stationary inboard portion of the brake caliper, with only the outboard caliper arm having to move toward or away from the brake disc permits simpler, lower cost and fewer-part caliper mounting arrangements, in part by eliminating the need to provide sliding caliper mounting arrangements. For example, the inboard portion of the caliper may be mounted in a fixed manner to the brake caliper mount, eliminating the need for precision-formed floating caliper mounting pins and bushings and their associated lubrication and environmental protection requirements.
A further advantage of the inventive rack-and-pinion brake application mechanism approach is the ability to precisely control the amount of brake application and retraction of both sides of the brake pad pair, lessening the potential for one or both of the brake pads to continue to apply a parasitic brake pad drag following release of the brake by the driver.
The rack-and-pinion rotary lever brake application mechanism design further has the advantage of being suitable for use of a variety of brake actuators, including pneumatic, electric and hydraulic actuators, as long as the brake application forces generated by the chosen actuator and the gearing ratios of the rotary lever arm, pinion and rack gearing permit the actuator to apply the desired amount of brake pad application force to the brake disc to obtain the desired braking performance at the wheel.
The present rack-and-pinion rotary lever design is not limited to circular or linear racks and/or pinions. For example, the racks and the pinion may be arranged as linear or other non-circular components, such as elliptical racks with a corresponding pinion profile to provide non-linear brake pad application in response to the motion of the rotary lever.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.